The light-emitting diode (LED) is a solid state semiconductor device. A structure of the light-emitting diode (LED) comprises a p-type semiconductor layer, an n-type semiconductor layer, and a light-emitting layer. The light-emitting layer is formed between the p-type semiconductor layer and the n-type semiconductor layer. The structure of the LED generally comprises group III-V group compound semiconductor such as gallium phosphide, gallium arsenide, or gallium nitride. The light-emitting principle of the LED is the transformation of electrical energy to optical energy by applying electrical current to the p-n junction to generate electrons and holes. Then, the LED emits light when the electrons and the holes combine.